In many synthetic and analytical procedures, it is necessary to increase the concentration of a liquid solution sample by the controlled evaporation of the solvent. One commonly employed apparatus comprises the micro or Kuderna Danish concentrators disclosed, for example, by Shugar et al, Chemical Technicians Ready Reference Handbook, 2nd Ed., McGraw-Hill, New York (1981). These concentrators comprise glass flasks consisting of a relatively large volume solution vessel, for example, of 10 to 100 mL, above a relatively small volume cold finger trap, for example, of 0.5 to 1 mL. A reflux condenser is positioned at the upper end of the concentrator flask by means of a ground glass joint. An electric heating unit is commonly employed with these concentrators. The heating unit consists of an aluminum block provided with one or more holes that accommodate the solution vessel section of the concentrator, leaving the cold finger section exposed to ambient air, and therefore in thermally responsive relation to the ambient air. The solution vessel is heated by the block using proportionally controlled resistive heaters to provide the desired evaporation.
These known concentrators have several serious disadvantages. First, the electrically heated aluminum block poses a safety hazard. That is, if the solvent being evaporated is flammable and has a vapor density greater than that of air, vapors can accumulate at the base of the unit and ignition can occur by arcing in the controller or switch. Thus, such a unit is not suitable for use in an explosion-proof laboratory. The concentrators are also disadvantageous in that the cold finger trap in ambient air is often warmed significantly through conduction from the heating block and conduction through the solution vessel. This heating is undesirable since the cold finger section should be maintained at a cooler temperature to minimize bumping and decomposition of sensitive solutes. This heating is further undesirable since it applies an unfavorable temperature gradient opposing thermal solute transport. Furthermore, it is difficult to observe the liquid in the solution vessel section of the concentrator while the flask is in the heating block. Accordingly, a need exists for an improved evaporative concentration apparatus for use with liquid samples.
Examples of further prior art concentrating apparatus and methods are disclosed in the Senseman U.S. Pat. No. 1,651,557, the Krause U.S. Pat. 2,248,634, the Solomon U.S. Pat. No. 4,311,668 and the Friswell U.S. Pat. No. 4,707,452. Additionally, British Patent No. 1,118,161 discloses a laboratory gas generator apparatus including a liquid container having an enlarged middle portion and a lower, open ended finger portion.